Use whatever tires come with the bike until you've put it through its paces at your island. Then, if you find yourself sinking in sand or soft dirt often, you'll want the fattest tires that will fit on the bike. If you're riding more on roads, then a less knobby narrower tire.

It's always a trade off: knobbier tires give you traction on dirt but waste energy on paved roads. Fatter tires provide cushion and floatation on soft dirt, but are heavier. Feel the terrain out before getting new tires.